Document U.S. Pat. No. 4,620,267 in the name of the Applicant discloses a correction device which is mainly constituted by at least two photoelectric cells secured to the reflector of a headlight, the cells looking at the road at two different distances ahead of the vehicle. The variations in the brightness of the road illuminated by the headlight in the two directions along which the respective cells look are used in processing means for the purpose of correcting the elevation of the headlights as a function of variations in the attitude of the vehicle. In one embodiment, the processing means mathematically divide values of the instantaneous currents passing through the cells, and the result of the division is compared with a threshold value.
Nevertheless, such a prior art device suffers from the drawback of being extremely sensitive to changes in the nature of the road surface, in the transparency of the headlight glass, in the feed voltage to the lamp, and also in weather conditions (rain that makes the road surface shiny, fog, etc.).
More precisely, the above parameters co-operate to impart random multiplying coefficients to the brightness as measured by each cell; in practice a prior art corrector as defined above works properly only under perfect conditions concerning road surface, glass cleanliness, weather, etc.